1. Field of the Invention
The present invention generally relates to a refrigerator installing structure. More specifically, the present invention relates to a refrigerator installing structure of accommodating a refrigerator inside a sleeve provided inside a vacuum vessel.
2. Description of the Related Art
For example, a Gifford-McMahon refrigerator (hereinafter, referred to as a “GM refrigerator” has been widely used as a means for cooling a cryostat (a cryo temperature vacuum vessel) such as a superconducting magnet apparatus. When such a GM refrigerator is used for a long time, maintenance is required.
In a method of maintaining the superconducting magnet apparatus or the like, the entire superconducting magnet apparatus is required to have an ordinary temperature. It takes from one day to six days to cause the temperature of the superconducting magnet apparatus to reach an ordinary temperature. In the meantime, because the superconducting magnet apparatus is continuously stopped under an idle condition, running efficiency of the superconducting magnet apparatus is greatly reduced.
Meanwhile, there is proposed a method of extracting a displacer while a cylinder of the GM refrigerator is fixed to the vacuum vessel. Within the method, the cylinder is continuously exposed to the air and the cylinder is continuously cooled by the vacuum vessel. Therefore, moisture in the air changes to be an ice film and adheres to the inner surface of the cylinder. Therefore, it is impossible to insert the displacer again inside the cylinder. Consequently, the maintenance becomes impossible.
Then, as a maintenance method by which the superconducting magnet apparatus or the like is maintained while preventing the ice film from adhering to the inner surface of the cylinder, there is proposed a method of forming a space separated from a vacuum zone of a vacuum vessel and installing a cylinder of the GM refrigerator in the space (Patent Document 1).
In this structure, when the GM refrigerator is installed in the sleeve, a cooling stage of the GM refrigerator is connected to an object to be cooled located inside the vacuum vessel via the sleeve. Further, the space is formed between the sleeve and a flange of the GM refrigerator. The space is vacuated. Further a sealing member (an O-ring) is provided between the sleeve and the cylinder so as to maintain a degree of vacuum in the space.
In the above structure, when the superconducting magnet apparatus or the like is maintained, a cylinder is separated from the sleeve by a small amount, e.g., several mm, thereby releasing a thermal connection between the cylinder and the sleeve is released. However, even if the cylinder is moved relative to the sleeve, sealing by the O-ring is ensured to thereby maintain a vacuum in the space formed between the sleeve and the cylinder.
As described, because the vacuum space exists between the sleeve and the cylinder and the sleeve and the cylinder are thermally separated, a low temperature of the vacuum vessel does not thermally conduct from the cylinder to the inside of the vacuum vessel. Therefore, the ice film, which causes a problem in the maintenance, does not adhere to the inside of the cylinder. The displacer can be easily replaced within a short time.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2004-053068